1. Field of the Invention
The present invention relates to an ink jet recording apparatus and a discharge recovery method, and more particularly to the execution timing of discharge recovery processing executed for keeping the discharge performance of a recording head to be good.
2. Description of the Related Art
A recording apparatus having the functions of a printer, a copier, a facsimile, and the like, and a recording apparatus used as the output equipment of a computer, a word processor, a work station, and the like have been known. These recording apparatus are adapted to record an image (including a character, a sign, and the like) on a recording medium such as a sheet of paper and a plastic thin plate (such as an OHP sheet) based on image information. Among the recording apparatus of this kind, a serial type recording apparatus adopting a recording system of performing the main scan thereof in the direction intersecting with the conveyance direction of the recording medium (sub scan direction) is widely used. The system records an image with a recording head mounted on a carriage moving along the recording medium (main scan), and performs a predetermined quantity of paper feed (sub scan) after the end of the recording for one scan. By repeating the main scan of the recording head and the sub scan of the recording medium, the image can be recorded in a desired range on the recording medium.
The systems of the recording head include an ink jet system, a wire-dot system, a thermosensitive system, a thermal transfer system, a laser beam system, and the like. The ink jet system among them discharges ink from a recording head to a recording medium to perform recording, and can easily make the recording head compact and can record a highly fine image at a high speed. Moreover, the ink jet system has the following advantages: the system can perform recording on a sheet of plain paper without needing any special processing, the running cost thereof is inexpensive, noises are little owing to being a non-impact system, and a color image can be easily recorded using multi-color ink, and the like.
The recording head of the system of discharging ink using thermal energy among the ink jet systems is especially manufactured by forming an electrothermal transducer, electrodes, liquid path walls, a top plate, and the like on a substrate through a semiconductor manufacturing process including etching, vapor deposition, sputtering, and the like. From this manufacturing method, the recording head having a high density discharge port arrangement can be easily manufactured, and the recording head can be formed to be more compact. Moreover, the recording unit of the recording head can be easily formed to have a long size or to be a plane (to have two dimensions) by utilizing the advantages of an IC technology and a micro working technology, and the recording unit can be also easily fully multiplied and mounted in high density.
The ink jet recording head like this arranges its discharge ports at a pitch of, for example, 1/600 inches or 1/2400 inches. In the recording head like this, ink sometimes attaches on a discharge port surface owing to ink mist produced at the time of ink discharge or splashes produced by the impact produced when the discharged ink reaches the recording medium. In this case, the attached ink sometimes obstructs the discharge ports to generate a defective discharge. Accordingly a configuration for removing the ink attached to the circumferences of the discharge ports by wiping out the ink by providing a blade made of an elastic body such as rubber and moving the recording head with the blade abutting against the discharge port surface of the recording head. This is known as wiping, and one of the discharge recovery processings for keeping the discharge performance of the recording head in a good state.
Moreover, the so-called preliminary discharge is known as another mode of the discharge recovery processing. The preliminary discharge performs an ink discharge that does not participate in the recording of an image from the recording head at a predetermined position of the apparatus. A thickened ink is ejected from the inside of an ink flow path to prevent the defective discharge beforehand. At the time of recording, ink is selectively discharged from a plurality of discharge ports of the recording head to form an image. If individual discharge ports are examined, some of them do not perform any discharges of ink according to some image data, so that the ink remains being exposed to the open air to some image data. The viscosity of the ink remaining in the discharge ports like this increases, and thereby a defective discharge such as the decrease of the amount of discharged ink and the deflection of a discharge direction may occur. The defective discharge like this can be prevented beforehand by performing the preliminary discharge periodically.
The so-called absorption recovery processing is known as a still other mode of the discharge recovery processing. The recovery processing absorbs ink and ejects the ink from the discharge ports by capping the discharge port surface of the recording head and producing a negative pressure in the cap. The recovery processing ejects the bubbles staying particularly in an ink path and a common liquid chamber together with ink.
The timing of executing the discharge recovery processing mentioned above is frequently based on factors such as a discharge frequency and an environmental temperature, and the discharge recovery processing is executed at the timing when the factors satisfy the conditions in which the recovery processing becomes necessary. Japanese Patent Application Laid-Open No. H09-207358 describes one example of the discharge recovery processing. The Japanese Patent Application Laid-Open No. H09-207358 describes that the interval between the performance of the preliminary discharge is determined according to the degree of temperature rise of a recording head. Moreover, the Japanese Patent Application Laid-Open No. H09-207358 describes that the number of times (the number of dots) of the discharges for one page is counted every end of recording for one page, and that, if the number of times is a predetermined value or more, absorption recovery is performed. Moreover, the Japanese Patent Application Laid-Open No. H09-207358 also describes that the counted number of dots is corrected at this time according to an environmental temperature and the degree of temperature rise.
Now, as described above, the determination of the execution timing of the conventional discharge recovery processing is uniformly executed when the discharge recovery processing satisfies the necessary condition. For example, if the degree of temperature rise is that of making the thickening of ink reach a value near to the limit of producing a defective discharge, the preliminary discharge is executed. Or, if discharges have been performed by the number of dots at which it can be presumed that a staying bubble has grown to the size that influences a discharge, the absorption recovery is executed.
However, such method of determining the execution timing of the discharge recovery processing is uniform, and as a result, unnecessary recovery processing is sometimes performed. In particular, the degree of the contamination of the discharge port surface of a recording head, which is an object of wiping, relatively changes owing to the duty (density) of recording dots and a drive frequency. Consequently, wiping is sometimes preformed at unnecessary timing in some contaminated states of the discharge port surface.
As a result, the conventional discharge recovery has a problem with the lowering of throughput by the relatively frequent wiping operations. Moreover, there are some cases where the frequent wiping operations cause deterioration of the image quality of a recorded image. That is, if the wiping is frequently performed, there are some cases where a difference in the density or the tint that can be produced between the images of the bands formed by the scans before and after the wiping becomes remarkable to deteriorate the image quality. In concrete terms, the time interval from the end of the scan just before wiping to the start of the scan just after the wiping is longer than the time interval of the scans without any wiping operations put between them, and an image that has a part where the density or the tint is different from that in the other parts according to the longer time interval is formed. For example, in the case of one path recording, stripe-like density unevenness can be produced at the boundary between the bands before and after wiping, or the degree of generation of the stripe-like density unevenness can differ from that at the other parts. Moreover, especially in the one-path recording, the influences exerted on the following scans by cockling, which can be produced by a scan, can become larger after the elapsing of a relatively long time with intervening wiping. The difference of density may be produced by the influences of the cockling. Moreover, in multi-path recording for performing recording in a determined region by a plurality of times of scans, the time interval between scans before and after wiping is similarly longer than that of scans without intervening any wiping operations, and an image that has a part where the tint is different from that in the other parts according to the longer time interval is formed.
As described above, the discharge recovery processing such as wiping especially has a problem with density unevenness, a tint, and the like caused by the discharge recovery processing if the discharge recovery processing is simply uniformly executed at the timing satisfying the conditions necessary for the discharge recovery processing.